Method for manufacturing parts with a low waviness from an electrogalvanized metal sheet, corresponding part and vehicle

ABSTRACT

A method for manufacturing parts is provided. The method includes cold rolling a substrate with work cylinders whose work surface has a roughness Ra 2.5  of less than or equal to 3.6 μm, depositing the metal coating on at least one face of the annealed substrate by electrodeposition to form the metal sheet and deforming the cut metal sheet to form the parts. The outer surface of the metal coating has a waviness Wa 0.8  of less than or equal to 0.5 μm after the deformation step. A part and vehicle are also provided.

The present invention relates to a method for manufacturing a metalsheet comprising a substrate, at least one face of which is covered by azinc-based metal coating.

BACKGROUND

Such a metal sheet is more particularly designed to manufacture bodyparts for a land-based motorized vehicle, such as a motor vehicle.

The metal sheet is then cut and deformed to produce parts that areassembled to form the body or shell.

That shell is next coated with a film of paint (or paint system), whichprovides a good surface appearance and, with the zinc-based metalcoating, helps protect against corrosion.

Zinc-based coatings for metal sheets have what is referred to as awaviness of their outer surfaces, which currently can only be offset bysignificant paint thicknesses, failing which there may be an “orangepeel” appearance that is unacceptable for body parts.

The waviness W of the outer surface of a coating is a gentle,pseudo-periodic geometric irregularity, with a relatively longwavelength (0.8 to 10 mm), that differs from the roughness R, whichcorresponds to geometric irregularities with short wavelengths.

BRIEF SUMMARY

In the present invention, the arithmetic mean Wa of the wavinessprofile, expressed in μm, has been chosen to characterize the wavinessof the outer surface of a metal sheet coating, and the wavinessmeasurements have been conducted in accordance with standard SEP 1941,1^(st) edition dated May 2012, in particular with a cutoff threshold of0.8 mm. These measurements are deemed compliant with this standard byWa_(0.8).

A decrease in the waviness Wa_(0.8) may make it possible to reduce thethickness of the paint film used to achieve a given painted appearancequality or, at a constant paint film thickness, to improve the qualityof the paint appearance.

An object of the present invention provides a method for manufacturingparts by cutting and forming a metal sheet, the metal sheet comprising asubstrate whereof at least one face has been coated with a zinc-basedmetal coating, the outer surface of the metal coating having a reducedwaviness Wa_(0.8) after deformation.

The present invention provides a method for manufacturing parts madefrom a metal sheet comprising a substrate whereof at least one face iscoated with a zinc-based metal coating, the method comprising at leastthe following steps:

providing the substrate,

cold rolling the substrate, during which step at least the last coldrolling pass is done with work cylinders whose work surface have aroughness Ra_(2.5) of less than or equal to 3.6 μm,

recrystallization annealing of the cold rolled substrate,

skin-pass of the annealed substrate,

deposition of the metal coating, on at least one face of the annealedsubstrate, by electrodeposition to form the metal sheet,

cutting the metal sheet, and

deforming the cut metal sheet to form the parts, the outer surface ofthe metal coating having a waviness Wa_(0.8) of less than or equal to0.5 μm after the deformation step.

The method may also comprise the following features, considered alone orin combination:

at least the last cold rolling pass is carried out with work cylinderswhose work surfaces have a roughness Ra_(2.5) of less than or equal to3.3 μm;

at least the last cold rolling pass is carried out with work cylinderswhose work surfaces have a roughness Ra_(2.5) of less than or equal to3.2 μm;

at least the last cold rolling pass is carried out with straightened andnon-etched work cylinders whose work surfaces have a roughness Ra_(2.5)of less than or equal to 0.5 μm;

at least the last cold rolling pass is carried out with straightened andnon-etched work cylinders whose work surfaces have a roughness Ra_(2.5)of less than or equal to 0.4 μm;

at least the last cold rolling pass is carried out with work cylinderswhose work surfaces are either not etched, or stochastically etched;

the skin-pass step is performed with EDT work cylinders whose worksurfaces have a roughness Ra_(2.5) comprised between 1.65 and 2.95 μm;

the skin-pass step is performed with EDT work cylinders whose worksurfaces have a roughness Ra_(2.5) comprised between 1.65 and 2.30 μm;

the method comprising a step for painting the deformed parts;

the outer surface of the metal coating has a waviness Wa_(0.8) of lessthan or equal to 0.45 μm after the deformation step;

the outer surface of the metal coating has a waviness Wa_(0.8) of lessthan or equal to 0.40 μm after the deformation step; and

the outer surface of the metal coating has a waviness Wa_(0.8) of lessthan or equal to 0.35 μm after the deformation step.

The present invention also provides a part which may be obtained usingthe method as defined above, wherein the outer surface of the metalcoating has a waviness Wa_(0.8) of less than or equal to 0.50 μm.

The part may also comprise the following features, considered alone orin combination:

the outer surface of the metal coating has a waviness Wa_(0.8) of lessthan or equal to 0.45 μm;

the outer surface of the metal coating has a waviness Wa_(0.8) of lessthan or equal to 0.40 μm;

the outer surface of the metal coating has a waviness Wa_(0.8) of lessthan or equal to 0.35 μm; and

the part further comprises a paint film on the metal coating.

The invention also provides a motorized land-based vehicle comprising abody, the body comprising a part as defined above.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be illustrated by examples provided for informationalpurposes, and non-limitingly, in reference to the appended FIGURE, whichis a diagrammatic sectional view illustrating the structure of a metalsheet according to the invention.

DETAILED DESCRIPTION

The sheet 1 comprises a steel substrate 3, each of the two faces 5 ofwhich are covered by a metal coating 7.

It will be noted that the relative thicknesses of the substrate 3 andthe different layers covering it has not been respected in FIG. 1 inorder to facilitate the illustration.

The coatings 7 present on the two faces 5 are generally similar, andonly one will be described in more detail below. As an alternative, onlyone of the faces 5 has a coating 7.

The coating 7 generally has a thickness smaller than or equal to 25 μmand aims to protect the substrate 3 from corrosion.

The coating 7 is a zinc-based coating. It may for example use zinc, or azinc alloy, such as zinc-nickel or zinc-iron or zinc-cobalt, or azinc-polymer composite.

The following method may for example be used to manufacture the metalsheet 1.

A substrate 3 in the form of a strip is used, for example obtained byhot rolling.

The substrate 3 first undergoes a cold rolling step.

Preferably, for the cold rolling, one begins by rolling the substrate 3with a reduction rate generally comprised between 60 and 85%, so as toobtain a substrate 3 with a thickness for example comprised between 0.2and 2 mm.

Care will be taken so that at least the last cold rolling pass is donewith work cylinders with a reduced thickness, i.e., the work surfaces ofwhich have a thickness Ra_(2.5), i.e., measured with a cutoff thresholdat 2.5 mm, of less than or equal to 3.6 μm, and preferably less than orequal to 3.3 μm or even 3.2 μm.

It will be recalled that the work cylinders are the cylinders of theroller in direct contact with the substrate 3 to deform it. The term“work surface” designates their surfaces in contact with the substrate3.

The work cylinders with reduced roughness are present at least in thelast cage(s) of the roller in the advance direction of the substrate 3in the roller.

Preferably, the work surfaces are not etched, or are etchedstochastically, i.e., using a method creating a stochastic texture. Sucha method may for example be an EDT (Electro Discharge Texturing) method.EBT (Electron Beam Texturing) methods are ruled out in this case, sincethey produce a non-stochastic texture.

In one alternative, the work cylinders are so-called “smooth” workcylinders, i.e., straightened and non-etched cylinders, the worksurfaces of which have a roughness Ra_(2.5) smaller than or equal to 0.5μm, or even 0.4 μm.

The work cylinders with reduced roughness positioned on either side ofthe substrate 3 will preferably have been made using the same method andwill therefore have the same characteristics.

The cold rolled substrate 3 can next undergo annealing carried outconventionally in an annealing furnace under an appropriate atmosphere,in order to recrystallize it after the work hardening it has undergoneduring the cold rolling operation.

The recrystallization annealing also makes it possible to activate thefaces 5 of the substrate 3 in order to favor the chemical reactionsnecessary for the subsequent electrodeposition operation.

Depending on the grade of the steel, the recrystallization annealing isdone at a temperature comprised between 650 and 900° C. for the timenecessary to recrystallize the steel and activate faces 5.

The substrate 3 next undergoes a skin-pass operation to give the faces 5a texture facilitating subsequent shaping of the metal sheet 1.

In fact, the skin-pass operation makes it possible to transfer asufficient roughness to the faces 5, and therefore the outer surfaces 21of the coatings 7 of the metal sheet 1, so that it can subsequently beshaped under good conditions, while favoring good retention of the oilapplied to the metal sheet 1 before it is shaped.

The elongation rate of the substrate 3 during the skin-pass operation isgenerally comprised between 0.5 and 2%.

In one alternative, the skin-pass operation will be carried out with EDTwork cylinders whose work surfaces have a roughness Ra_(2.5) comprisedbetween 1.65 and 2.95 μm, preferably between 1.65 and 2.30 μm.

The coatings 7 are next made by electrodeposition. To that end, thesubstrate 3 passes through an electrolyte bath.

The progression speed of the substrate 3 in the electrolyte bath isgenerally comprised between 20 and 200 m/mn.

The electrolyte is for example an aqueous acetyl electrolyte of thesulfate, chloride or chloride-sulfate mixture type, suitable forelectrolytic deposition on the substrate 3 of either a layer of zinc, ora layer of zinc alloy, or a layer of zinc-polymer composite.

Thus, in order to deposit a zinc coating on the substrate 3, it ispossible to use a chloride-based electrolyte bath comprising 50 to 150g/l of zinc in the form of zinc chloride (ZnCl₂), 250 to 400 g/l ofpotassium chloride (KCl) and having a pH comprised between 4 and 5. Thebath can also comprise additives, for example grain refiners with aconcentration comprised between 1 and 1.5 m/l. In order to deposit thelayer of zinc on the substrate 3, the current density is preferablyadjusted to a value comprised between 30 and 150 A/dm², and thetemperature of the bath is preferably adjusted to a value comprisedbetween 40 and 80° C., and preferably below 60° C. to limit evaporationof the bath.

It is also possible to use a sulfate-based electrolyte bath comprising50 to 150 g/l zinc in the form of zinc sulfate (ZnSO₄), and having a pHbelow 5. The bath can also comprise additives, for example NaCO₃.Preferably, the current density is adjusted to a value comprised between10 and 150 A/dm², and the temperature of the bath to a value comprisedbetween 40 and 80° C., preferably below 60° C. to limit the evaporationof the bath.

If one wishes to deposit a zinc alloy coating, such as zinc-nickel,zinc-iron or zinc-cobalt, nickel, iron or cobalt ions are added to thesulfate-, chloride- or chloride-sulfate mixture-based electrolyte bathpreviously described.

Likewise, to deposit a zinc-polymer composite coating, 0.1 to 2 wt % ofa polymer, for example polyethylene glycol or polyacrylamide, is addedto the sulfate-, chloride- or sulfate-chloride mixture-based electrolytebath previously described. The obtained composite coating impartsexcellent corrosion resistance and also makes it possible to avoid thetoxic chromating or phosphating treatments necessary for the paint tocatch.

The surfaces 21 of the coatings 7 will preferably have referencesRa_(2.5) comprised between 0.9 and 1.8 μm, and still more preferablybetween 0.9 and 1.5 μm.

The metal sheet 1 thus obtained can next be cut, then shaped, forexample by stamping, bending or profiling, to form parts that can nextbe painted to form a paint film (or paint system), not shown, on eachcoating 7.

In the event of parts for household appliances, the paint films may alsobe subject to annealing by physical and/or chemical means, known inthemselves.

To that end, the painted part may be passed through a hot air orinduction oven, or under UV lamps or under a device diffusing electronbeams.

After deformation, the outer surfaces 21 of the coatings 7 of the parthave a waviness Wa_(0.8) of less than or equal to 0.50 μm, preferablyless than or equal to 0.45 μm, still more preferably less than or equalto 0.40 μm, or even 0.35 μm.

The use of work cylinders with a reduced roughness Ra_(2.5) at least forthe last cold rolling pass makes it possible to better control thewaviness Wa_(0.8) of the metal sheet 1 subsequently obtained by coatingthe substrate 3 on the one hand, and of the parts that can be producedby cutting and deforming the metal sheet 1 on the other hand.

In particular, such cold rolling makes it possible to reduce thewaviness Wa_(0.8) relative to rolling only using cylinders with a higherroughness.

Thus, the use of work cylinders with a roughness Ra_(2.5) of less thanor equal to 3.6 μm makes it possible to achieve a waviness Wa_(0.8) ofless than or equal to 0.50 μm after the coating deposition step or afterany deformation step.

The use of work cylinders with a roughness Ra_(2.5) of less than orequal to 3.3 μm, or even 3.2 μm, for example makes it possible toachieve a waviness Wa_(0.8) of less than or equal to 0.45 μm after thecoating deposition step or after any deformation step.

The use of smooth work cylinders with a roughness Ra_(2.5) of less thanor equal to 0.5 μm for example makes it possible to achieve a wavinessWa_(0.8) of less than or equal to 0.40 μm, or even 0.35 μm after thecoating deposition step or after any deformation step.

For automobile applications, after phosphating, each part is soaked in acataphoresis bath, and a primer layer, a base paint layer, andoptionally a finishing varnish layer are successively applied.

Before applying the cataphoresis layer on the part, the latter is firstdegreased, then phosphated so as to ensure adherence of thecataphoresis.

The cataphoresis layer then provides additional corrosion protection forthe part. The primer layer, generally applied using a gun, prepares thefinal appearance of the part and protects it from grit and UV rays. Thebase paint layer gives the part its final color and appearance. Thelayer of varnish gives the surface of the part good mechanical strength,the ability to withstand aggressive chemical agents and a good surfaceappearance.

Generally, the weight of the phosphating layer is comprised between 1.5and 5 g/m².

The paint films used to protect and guarantee an optical surfaceappearance for the parts for example comprise a cataphoresis layer witha thickness of 15 to 25 μm, a primer layer with a thickness of 35 to 45μm, and a base paint layer with a thickness of 40 to 50 μm.

In the event the paint film layers further comprise a varnish layer, thethicknesses of the different paint layers are generally as follows:

cataphoresis layer: between 15 and 25 μm, preferably less than 20 μm,

primer layer: less than 45 μm,

base paint layer: less than 20 μm, and

varnish layer: less than 55 μm.

The paint films may also not comprise a cataphoresis layer, and onlycomprise a primer layer and a base paint layer, and optionally a varnishlayer.

Preferably, the total thickness of the paint films will be less than 120μm, or even 100 μm.

The invention will now be illustrated by trials provided for informationand non-limitingly.

The trials done seek to show the positive influence of cold rolling donewith work cylinders with a reduced roughness Ra_(2.5), relative torolling done with cylinders whose work surface has a greater roughness.

To that end, grade DC-06 steel substrates undergo cold rolling to reacha thickness of 0.8 mm, using either EDT etched work cylinders, the worksurfaces of which have roughnesses Ra_(2.5) of 3.1 μm, 3.5 μm and 3.75μm, or smooth work cylinders whose work surfaces have a roughnessRa_(2.5) of 0.3 μm.

After recrystallization annealing at 720° C., the substrates 3 nextundergo a same skin-pass operation done with EDT etched work cylinders,the work surfaces of which have a roughness Ra_(2.5) of 1.85 μm.

The substrates 3 are next coated with zinc by electrodeposition in asulfate-based electrolyte bath. The metal sheets thus obtained are cutand shaped by 3.5% equi-biaxial deformation using a Marciniak tool.

The waviness Wa_(0.8) and roughness Ra_(2.5) values of the outersurfaces 21 of the coatings 7 are noted at the end of theelectrodeposition step (EG) and the deformation step (DEF).

The measurement results for Wa_(0.8) and Ra_(2.5) are provided in tableI. Trials 2 to 4 correspond to different embodiments of the inventionand are identified by asterisks.

TABLE I Difference Ra_(2.5) (μm) Wa_(0.8) cylinders Wa_(0.8) (μm)Ra_(2.5) (μm) Wa_(0.8) (μm) Ra_(2.5) (μm) after DEF and Trials for LAFwork after EG after EG after DEF after DEF after EG 1 3.75 0.47 1.120.55 1.24 +17% 2 3.5* 0.49 1.39 0.47 1.27 −4.1%  3 3.1* 0.36 1.27 0.360.96  0% 4 0.3* 0.40 1.05 0.35 0.86 −13%

It is in particular observed that the waviness Wa_(0.8) afterdeformation of the metal sheets according to the invention is close toor below the waviness level before deformation. This effect isparticularly clear for trials 2 and 4.

What is claimed is:
 1. A method for manufacturing parts made from ametal sheet comprising the steps of: providing a substrate; cold rollingthe substrate, at least a last cold rolling pass being done with workcylinders whose work surface have a roughness Ra_(2.5) of less than orequal to 3.6 μm; recrystallization annealing of the cold rolledsubstrate; skin-pass rolling the annealed substrate; depositing azinc-based metal coating on at least one face of the substrate byelectrodeposition to form a metal sheet; cutting the metal sheet; anddeforming the cut metal sheet to form parts, an outer surface of themetal coating having a waviness Wa_(0.8) of less than or equal to 0.5 μmafter the deformation step.
 2. The method according to claim 1, whereinat least the last cold rolling pass is carried out with work cylinderswhose work surfaces have a roughness Ra_(2.5) of less than or equal to3.3 μm.
 3. The method according to claim 2, wherein at least the lastcold rolling pass is carried out with work cylinders whose work surfaceshave a roughness Ra_(2.5) of less than or equal to 3.2 μm.
 4. The methodaccording to claim 3, wherein at least the last cold rolling pass iscarried out with straightened and non-etched work cylinders whose worksurfaces have a roughness Ra_(2.5) of less than or equal to 0.5 μm. 5.The method according to claim 4, wherein at least the last cold rollingpass is carried out with straightened and non-etched work cylinderswhose work surfaces have a roughness Ra_(2.5) of less than or equal to0.4 μm.
 6. The method according to claim 1, wherein at least the lastcold rolling pass is carried out with work cylinders whose work surfacesare not etched.
 7. The method according to claim 1, wherein at least thelast cold rolling pass is carried out with work cylinders whose worksurfaces are stochastically etched.
 8. The method according to claim 1,wherein the step of skin-pass rolling is performed with electrodischarge texturing work cylinders whose work surfaces have a roughnessRa_(2.5) of from 1.65 to 2.95 μm.
 9. The method according to claim 8,wherein the step of skin-pass rolling is performed with electrodischarge texturing work cylinders whose work surfaces have a roughnessRa_(2.5) of from 1.65 to 2.30 μm.
 10. The method according to claim 1,further comprising a step of: painting the deformed parts.
 11. Themethod according to claim 1, wherein the outer surface of the metalcoating has a waviness Wa_(0.8) of less than or equal to 0.45 μm afterthe deformation step.
 12. The method according to claim 11, wherein theouter surface of the metal coating has a waviness Wa_(0.8) of less thanor equal to 0.40 μm after the deformation step.
 13. The method accordingto claim 12, wherein the outer surface of the metal coating has awaviness Wa_(0.8) of less than or equal to 0.35 μm after the deformationstep.